The mass balance is one of the primary tools of engineering. Careful tracking of the materials consumed and produced enables one to adequately design for a process.
A simple example of a chemical mass balance would be hydrogen and oxygen combining to form water (H2O). From the chemical formula of water, it can be predicted that two moles of hydrogen are needed for every one mole of oxygen used in the reaction. Any more than two moles of hydrogen would result in an excess of hydrogen. Any less than two moles of hydrogen would result in an excess of oxygen. At the end of the process, the amount of water formed can be observed, and an accurate measurement of the reactants used (hydrogen and oxygen) could then be determined.
Because mass cannot be created or destroyed, the mass at the beginning must be equal to the mass at the end. The exercise of modeling what happens to the mass through the course of the reaction is called a mass balance.
The process outlined in this patent involves a mass balance on the reactants through the course of a batch fermentation. The complicating feature of a batch fermentation is that the food that is used by the yeast (or other microorganisms) is not homogeneous. The yeast utilize the best food first. The ‘best’ food is described as the fraction of the total that yields the most new yeast mass.
The yield of the yeast mass from the food can be plotted as a function of time (or food fraction). This yield curve usually shows that yeast can turn carbon from the food into more yeast mass with a 30% efficiency at first. (0.3 of the food weight ends up as yeast mass.) This percentage then gradually decreases over time as the food fraction becomes more difficult to use.
This yield curve is the key to the mass balance of the fermentation. It can be employed in a dynamic fashion to show the progress of the fermentation. If the starting concentrations (reactants) are known, only the progress of one of the products need be monitored to complete the mass balance, and specify the concentrations of all of the chemical species in the entire reaction.
In this way, the course of a batch fermentation can be modeled from only a continuous carbon dioxide measurement. Measuring devices for the carbon dioxide evolution from the fermentation are called ‘mass-flow’ devices, and are available with various sensitivities and for various average flow rates. They can be acquired from scientific instrumentation catalogs such as Cole-Parmer™ and others. The output signal from these mass-flow meters is analog, and must be converted to digital information for use by a computer. I am using a analog-to-digital converter card from a company called: Data Translation™. There are many other companies that offer similar devices.
The software for this application is being developed for the PC platform using the c++computer language. I am using Microsoft's™ Visual c++™ compiler, the industry standard at present.
The following details the modeling methodology for monitoring yeast growth from carbon dioxide (CO2) evolution. It is important to note that this same approach can be utilized for batch processes in which alcohol is not the end product (such as acetone production). It begins with a description of how a batch of wort (sterilized malt sugars) is successfully inoculated with yeast. These yeast proceed to ferment the sugars in the solution, producing additional yeast mass, alcohol, and carbon dioxide.
Defining the relationship mathematically between the production of yeast, alcohol, and carbon dioxide as the batch progresses enables the prediction of the other species as long as one of them is measured. This relationship is outlined. Finally, the results of an experimental batch are compared to the predictions from the model. The results show this approach provides a valuable tool for routine monitoring and characterization of batch processes.
This invention relates to a process for monitoring, characterization, and control of batch fermentation processes. The production of alcoholic beverages, acetone, and many pharmaceuticals are examples of batch fermentation processes. Other inventions that utilize computer monitoring/control of fermentation processes involve attempting to keep one or two of the variables constant with a computer-controlled feed pump (e.g. U.S. Pat. No. 4,856,421). This is done in an attempt to make a batch process into a continuous-flow process. This approach is a ‘mechanical’ type of control. No attempt is made to describe the underlying chemical processes through mass balance calculations.